Conventionally, a variety of printing methods such as lithographic printing (including normal lithography which uses a dampening solution and waterless lithography which uses no dampening solution), relief printing, intaglio printing and stencil printing are used to obtain all manner of printed items, including printed items on cardboard and various types of books, printed packaging materials such as form printed items, various plastic printed items, printed items for stickers and labels, and metal printed items, and inks best suited to each of these printing methods are used in the printing process. One such ink that is well known is the actinic ray-curable ink.
Actinic ray-curable inks include an unsaturated compound having actinic ray curability such as an acrylate ester compound as an ink component, and cure instantaneously upon irradiation with actinic rays, with a three dimensional crosslinking reaction of the unsaturated compound forming a tough coating film. In addition, because of this curability, post processing can be performed immediately following printing, resulting in improved productivity compared with conventional oxidative polymerization type inks. Further, actinic ray-curable inks also tend to be used widely in the field of printing for packaging, which requires good rub resistance in order to protect the design aesthetics. In such cases, lithographic printing is widely used, as it exhibits excellent print quality in addition to this improved productivity and reduced cost.
Furthermore, within packaging printing applications, the majority of applications relate to the field of foodstuff packaging. In this particular field, in addition to improvements in the print quality relating to the design aesthetics of the packaging, recent enhanced awareness of safety issues has resulted in increased discussion relating to the migration of raw materials from the coating composition into the food contents. This increased discussion of the migration of coating composition raw materials has naturally tended to be more pronounced in the European and American markets where safety awareness has generally been at a higher level, and Switzerland has recently enacted the world's first legal regulation relating to printing inks for foodstuff packaging (Swiss Ordinance (RS817.023.21). The effect of this ordinance has spread widely, not only throughout the whole of Europe, but also within other developed nations such as Japan, and other developing nations.
The content of this ordinance prohibits the use of substances having a certain degree of toxicity or for which concern has been reported in relation to environmental hormones as ink raw materials, and strictly restricts the amount of migration of the various raw materials used into the food contents. The amounts permitted by these migration restrictions are extremely small, in the order of ppb, making the design of inks extremely demanding. Moreover, large foodstuff manufacturers and foodstuff packaging manufacturers across the entire global market have now started adopting regulations of a similar standard to Swiss Ordinance (RS817.023.21) as their own internal regulations, meaning this level of low migration is now being demanded for all inks used for foodstuff packaging.
As illustrated by technology including Patent Documents 1, 2 and 3, research is being conducted from various viewpoints with the aim of achieving low migration, low odor or low bleeding characteristics. In Patent Document 1, by performing chemical bonding between a photopolymerization initiator or a sensitizer and a polyester resin, the amount used of a high-migration photopolymerization initiator or sensitizer can be reduced, and therefore migration, elution or evaporation of the photopolymerization initiator or sensitizer following the curing reaction can be reduced. Further, in Patent Document 2, reductions in the odor and migration are achieved by appropriate selection of the photopolymerization initiator in an actinic ray-curable ink. Moreover, in Patent Document 3, by using a novel photopolymerization initiator based on a thioxanthone backbone that is used as a sensitizer, a combination of superior curability and low bleeding characteristics is achieved.
However, if the test conditions used in the evaluations of low migration, low odor and low bleeding in Patent Documents 1 to 3 are analyzed, then in Patent Document 1, a film substrate is placed on the printed item, compression is performed at a temperature of 70° C. and a pressure of 100 kgf/cm2 for 5 hours, and the amounts of unreacted initiator and sensitizer that have migrated into the film are measured using a UV-VIS absorption spectrophotometer, whereas in Patent Document 2, in a similar manner, a film substrate is placed on the printed item, and following standing for 24 hours at a temperature of 60° C. under a pressure of 100 kg/cm2, the film is immersed in distilled water (temperature: 25° C., 24 hours), and the amounts of unreacted initiator and sensitizer, or unreacted monomers and oligomers, are measured using a UV-VIS absorption spectrophotometer. Further, in Patent Document 3, an unprinted white sheet of paper is placed on the printed item, and following standing for 24 hours at a temperature of 60° C. under a pressure of 15 g/cm2, the amounts of the initiator and sensitizer are measured.
Patent Documents 1 and 3 use measurement of the ultraviolet absorption spectrum of the film, and therefore quantitative measurement at the ppb order is difficult, and in Patent Document 3, the compression conditions and the standing time are slight. Moreover, in Patent Document 2, the film into which migration of the ink raw materials from the printed item is assumed to have occurred is extracted using distilled water, but in the contents of most regulations including Swiss Ordinance (RS817.023.21), the use of a high-concentration alcohol aqueous solution is deemed the most suitable for replicating the actual contents of foodstuff packaging. Moreover, the temperature and standing time conditions used in Patent Document 3 do not fully satisfy market demands. In addition, the majority of radical polymerizable monomers and oligomers used in actinic ray-curable inks lack a light absorption wavelength, meaning detection of these compounds using the type of UV-VIS absorption spectrophotometer described in Patent Document 2 is impossible.
Patent Documents 1 and 3 relate mainly to a reduction in the migration of the photopolymerization initiator or the sensitizer, and make no mention of a reduction in the migration of the radical polymerizable monomer or oligomer, which is the other main component in the crosslinking reaction.
Further, although methods such as those in Patent Documents 1 and 3 in which the molecular weight of the photopolymerization initiator or the sensitizer is increased are widely used as techniques for achieving low migration, low odor and low bleeding characteristics for the photopolymerization initiator or the sensitizer, the number of reactive groups per unit of molecular weight decreases, and the resulting problem of a deterioration in the curability has yet to be addressed.
Actinic ray-curable inks often use a combination of a photopolymerization initiator and a sensitizer to prevent any deterioration in the curability caused by the light-blocking effects of the organic or inorganic pigments contained within the ink. Patent Documents 1 and 2 both include a sensitizer as an essential component, but although the thioxanthone compounds used as representative examples of the sensitizer and the amino compounds disclosed in Patent Document 2 provide an improvement in the curability, they exhibit significant outward migration characteristics, meaning that when they are used as raw materials in ink compositions for printing foodstuff packaging, the probability of migration into the food contents is high. In Patent Document 3, because the basic backbone of the novel photopolymerization initiator is a thioxanthone structure, there is an inherent risk of residues such as unreacted matter.
In this manner, as enhanced awareness of safety issues relating to foodstuff and pharmnnaceutical packaging spreads around the world, there is growing demand for actinic ray-curable ink compositions that satisfy the standards of regulation such as Swiss Ordinance (RS817.023.21), but test conditions that fully recognize the actual production and storage states of products have yet to be formulated and established. Further, actinic ray-curable ink compositions that are capable of maintaining high levels of curability (high productivity) without using a sensitizer are also becoming increasingly necessary.